Industry has recently been required to achieve a reduction in amount of electric power consumption (consumption energy) (hereinafter abbreviated to “energy saving”) in various electrical appliances for use in production so as to reduce production costs. Further, an ordinary household has also been required to achieve energy saving in various home electronics (domestic electrical appliances) so as to reduce electric power rates.
Energy saving generally starts with a grasp of an actual state of electric power consumption (use of electric power). Thus, it has been considered to measure electric power by providing wattmeters to indoor wires or various electrical appliances in buildings such as a factory and a house. Generally, a clamp electric current sensor is used in such a wattmeter to measure an electric current. A conventional electric power measuring device or a conventional electric current measuring device is exemplified by electric power measuring devices or electric current measuring devices disclosed in Patent Literatures 1 through 3.
An electric current sensing device disclosed in Patent Literature 1 is arranged to cause a current transformer (CT) provided to a power line to sense a system electric current. According to the electric current sensing device, a power supply section of an electric current sensing calculation and monitoring section provided on the secondary side of the CT is provided with a rectifier circuit, a constant voltage DC output circuit, a backup power supply circuit, and a level converting circuit that are located in this order from the upstream side, the level converting circuit supplying electric power to the electric current sensing calculation and monitoring section by converting an output voltage of the constant voltage DC output circuit or the backup power supply circuit to a predetermined level. In a case where the output voltage of the constant voltage DC output circuit decreases, electric power is supplied from the backup power supply circuit. This allows a single CT to sense an electric current and supply electric power.
An electric power sensing device disclosed in Patent Literature 2 is included in a sensing target device that is driven by a three-phase four-wire system power supply. According to the electric power sensing device, a voltage measuring circuit includes no transformer and is constituted by a resistance voltage-dividing circuit, and the voltage measuring circuit is connected to three power lines of the respective phases other than a neutral line of the three-phase four-wire system power supply. Meanwhile, an electric current measuring circuit is electromagnetically-coupled with the three power lines by a current transformer. Electric power generated by a power supply section of the sensing target device is supplied to an electric power receiving circuit via an interface such as a connector, and the electric power is supplied from the electric power receiving circuit to each section of the electric power sensing device, so that the electric power sensing device starts operating.
The voltage measuring circuit senses a voltage between the neutral line and each of the power lines of the three phases, and the electric current measuring circuit senses respective electric currents of the three phases by use of the current transformer. An analog output, which is a result of the measurement by each of the voltage measuring circuit and the electric current measuring circuit is digitalized by an A/D converter, and a result obtained from the A/D converter is used by a calculation section to calculate, for example, a phase voltage, a phase electric current, phase electric power, and total electric power. A result obtained from the calculation section is supplied to a communication circuit, and the result is transmitted from the communication circuit to the sensing target device via the interface such as a connector.
According to an electric power usage providing device disclosed in Patent Literature 3, one (1) power line of an alternating-current power device is provided with three core coils, and an electric power sensing section uses induced electromotive force, which is generated by a first core coil, to calculate electric power used by the alternating-current power device. A sensing electric power generating section uses induced electromotive force, which is generated by a second core coil, or uses a primary battery to supply electric power to the electric power sensing section. A communication section uses induced electromotive force, which is generated by a third core coil, or uses the primary battery to transmit the used electric power to another communication device. By thus intermittently calculating and transmitting used electric power, used electric power can be stably calculated and transmitted for a long period even in the case of electric power supply by electromagnetic induction.